Abstract
Abstract. The shells of two marine bivalve species (Fulvia tenuicostata and Soletellina biradiata) endemic to south Western Australia have been characterised using a combined crystallographic, spectroscopic and geochemical approach. Both species have been described previously as purely aragonitic; however, this study identified the presence of three phases, namely aragonite, calcite and Mg-calcite, using XRD analysis. Data obtained via confocal Raman spectroscopy, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) show correlations between Mg ∕ S and Mg ∕ P in F. tenuicostata and between Sr ∕ S and S ∕ Ba in S. biradiata. The composition of the organic macromolecules that constitute the shell organic matrix (i.e. the soluble phosphorus-dominated and/or insoluble sulfur-dominated fraction) influences the incorporation of Mg, Sr and Ba into the crystal lattice. Ionic substitution, particularly Ca2+ by Mg2+ in calcite in F. tenuicostata, appears to have been promoted by the combination of both S- and P-dominated organic macromolecules. The elemental composition of these two marine bivalve shells is species specific and influenced by many factors, such as crystallographic structure, organic macromolecule composition and environmental setting. In order to reliably use bivalve shells as proxies for paleoenvironmental reconstructions, both the organic and inorganic crystalline material need to be characterised to account for all influencing factors and accurately describe the vital effect.
Highlights
As calcifiers, molluscs play an important role in the ocean carbonate cycle
The X-ray diffraction (XRD) patterns indicate that the shells are composed of calcite and aragonite based on matching diffraction peaks (Fig. 3)
Both species have been described as purely aragonitic, but low levels of calcite and Mg-calcite are present in S. biradiata and F. tenuicostata, respectively
Summary
Molluscs play an important role in the ocean carbonate cycle. The calcium carbonate formed by marine organisms is a complex geochemical source and sink of carbon, which controls total oceanic carbon content and pCO2 (partial pressure of CO2) and more generally contributes to ocean alkalinity (e.g. Gattuso and Hansson, 2011). Freitas et al, 2006; Gillikin et al, 2005b; Lowenstam and Weiner, 1989) and environmental conditions at the time of deposition Ferguson et al, 2011; Gazeau et al, 2010; Hahn et al, 2014; Heinemann, 2011; Henkes et al, 2013; Lowenstam and Weiner, 1989; Schöne et al, 2011). Molluscs are globally distributed and demonstrate sequential growth, providing high-resolution seasonal and sub-seasonal records of environmental conditions Ayling et al, 2006; Bonham, 1965) Many oceanic conditions, such as seawater surface temperature, productivity, circulation and carbon reservoir dynamics, have been reconstructed successfully using bivalve shell records
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